Floating structure, such as drill ships and semisubmersibles, have long been used to drill and complete subsea wells. Most of these floating structures have a large-diameter cylindrical opening, known as a moonpool, which extends vertically through the hull of the structure. The moonpool provides access from a work platform on the upper end of the structure into the water below. Most drilling and completion operations are carried out through the moonpool.
The work platform on a floating structure must remain relatively stable during the drilling and completion of a subsea well. In extremely deep waters or in areas subject to high waves and strong winds, providing a stable floating platform can be a problem. One type of floating structure which is particularly adapted for use in such environments is the caisson vessel. A typical caisson vessel has an elongated, cylindrically-shaped hull, formed of concrete. The vessel has ballast tanks or compartments on its lower end and a work platform at its upper end. When in an operable position, the hull is moored vertically in the water with a substantial portion of the hull being submerged.
The submerged portion of the hull of the caisson vessel is relied on to provide stability for the work platform. Once submerged, the bottom of the vessel may be as much as 500 feet below the surface of the water. Thus the moonpool, extending from the work platform completely through the hull, may also be 500 feet long. Unfortunately, it is difficult to raise and lower drill strings and equipment through long moonpools without damaging the moonpool walls or the equipment being handled. In order to prevent damage to the drill strings and equipment or the walls of the moonpool, a system must be provided for centering and guiding the various drill strings as they pass through the moonpool.
One operation which is sometimes carried out through a moonpool is the lowering of a robot tool on a drill string to perform various operations on equipment located on the marine bottom. Since the robot tool is remotely-controlled, an umbilical line for furnishing power to the tool is run along the length of the drill string. The umbilical line is banded or otherwise secured at intervals to the drill string. In guiding such a drill string with its attached umbilical line through the moonpool, the guidance system used must be capable of engaging both the drill string and the umbilical line without damaging the umbilical line. At the same time, the guidance system must allow free vertical movement of the drill string and umbilical line so that they can easily be raised or lowered, as desired.
Providing a guidance system for the drill string and umbilical line is further complicated by the requirement that the robot tool must be capable of rotation once it has reached the depth of the subsea equipment. This is necessary in order to allow the work elements on the robot tool to be properly oriented with respect to the equipment to be worked on. For example, a robot tool with a valve actuator must be rotated by thrusters on the tool to properly position the actuator with respect to the valve to be operated before the actuator can be engaged. Since the robot tool is affixed to the drill string, the drill string with its attached umbilical must also be capable of rotating. Thus the guidance system must allow the drill string to rotate while the system is engaging the drill string.